As well known, the performance of a cutting edge largely depends on into what shape the ridge of the cutting edge is processed. As the ridge becomes sharper, the cutting ability improves while the durability tends to deteriorate, such as a chip being more likely to occur on the ridge. In order to improve the durability or to prevent a material from being snagged on the ridge, it is common practice to round the ridge. For example, Patent Literature 1 (Japanese Patent Application Laid-Open No. 2008-112523) discloses an example in which an edge of a glass disk is rounded by wet etching. Patent Literature 2 (Japanese Patent Application Laid-Open No. 2005-224419) discloses an example in which the cutting edges of a pair of scissors are rounded by lapping.
Furthermore, it is also common practice to perform beveling, such as forming a facet or chamfer, in order to increase the strength of the ridge of the cutting edge. For example, Patent Literature 3 (Japanese Patent Application Laid-Open No. 2004-58168) discloses an example in which a chamfer is formed on a cutting edge. If the beveling is performed to form the facet or the chamfer, the durability can be improved without significantly deteriorating the cutting ability.
As a method of precisely polishing areas smaller than the areas to which the wet etching and the lapping can be applied, a method of using a gas cluster ion beam has been proposed. Patent Literature 4 (Japanese Patent Application Laid-Open No. 2011-512173) discloses an example in which a surgical scalpel is irradiated with a gas cluster ion beam to sharpen the ridge thereof. The energy of the gas cluster ion beam is more concentrated in the vicinity of the surface of the material than the monomer ion beam, and therefore, the gas cluster ion beam has an advantage that it can achieve low-damage processing. Therefore, even an extremely sharp cutting edge can be processed without doing damage, such as small cracks, to the cutting edge.
Furthermore, Patent Literature 5 (Japanese Patent Application Laid-Open No. 2010-36297) discloses a result of irradiation of a cutting edge with a gas cluster ion beam. This literature proposes a method of using a gas cluster ion beam to smooth the surface of a diamond coating film the maximum height Rz of the profile in a 10-μm square of which is greater than 1 (Rz is defined according to Japanese Industrial Standards B0601:2001. The maximum height Rz of the profile is a sum of the maximum value of the profile peak height Zp from the average line of the contour curve and the maximum value of the profile valley depth Zv from the average line). As influences on the ridge of the cutting edge, it is shown that the chamfers originally formed on the diamond-coated tool remain, the ridge of the cutting edge is not rounded after the irradiation with the gas cluster ion beam, and the surface roughness does not significantly change even if the ridge of the cutting edge is perpendicularly irradiated with the gas cluster ion beam.
If a crystal material is etched by wet etching or monomer ion beam etching, a certain crystal face may selectively appear because of the anisotropy of the crystal material. In some cases, the anisotropy can be effectively used. However, if wet etching or monomer ion beam etching is used for a precise instrument, there arises a problem that the shape of the ridge of the cutting edge cannot be controlled as desired. In the case of an amorphous material rather than the crystal material, there is a problem that the etching may be nonuniform because of a phase separation or various defects in the material, and the nonuniformity significantly decreases the mechanical durability of the cutting edge.
Lapping or other similar art is a process of shaving the surface of the material of the cutting edge with abrasive grain and therefore inevitably does fine damage to the surface of the material of the cutting edge when shaving the surface with the abrasive grain and decreases the mechanical durability of the cutting edge.
If a facet or a chamfer (collectively referred to as a facet hereinafter) is formed on the ridge of the cutting edge for beveling, the mechanical durability may be improved to some extent. However, as far as this process is performed by using a conventional art, such as wet etching, monomer ion beam etching, laser beam machining or lapping, there is a problem that a small scratch or crack or a brittle affected layer occurs in the facet, and the adequate mechanical durability cannot be achieved.
The gas cluster ion beam art can achieve low-damage processing. However, only art for sharpening a cutting edge has been disclosed yet. If the cutting edge is sharpened, there is a problem that the mechanical durability of the cutting edge tends to be inadequate. Although the ridge of the cutting edge can be made blunt by using the gas cluster ion beam art, there is a problem that simply making the ridge of the cutting edge blunt increases the cutting resistance or otherwise decreases the cutting ability.
Patent Literature 5 discloses a result of irradiation of a cutting edge with a gas cluster ion beam as described above. According to Patent Literature 5, however, the surface of a diamond coating film the maximum height of the profile of which in a 10-μm square is greater than 1 μm is planarized with the gas cluster ion beam, and a facet cannot be formed on the surface even though the surface can be planarized. In addition, Patent Literature 5 does not propose any processing method that can precisely control the shape of a ridge of a cutting edge.
As described above, there has been proposed no conventional processing method that can precisely control the shape of a ridge of a cutting edge and form an ideal facet without doing even fine damage to a cutting edge.